Genetic engineering has the potential to correct
human genetic disorders. in gene therapy, a
defective gene is replaced by using a virus to
insert a normal gene into the cells of an
individual. this treatment will be most
successful if the virus is inserted into cells that
(1) lack a nucleus
(2) are recycled after death, rather than
removed from the body
(3) carry out one specific function, rather than
multiple functions
(4) continue to divide during the life of the
patient

Viruses do not have a nucleus, they cannot be recycled because they release their genetic code and that cannot be recovered unless they replicate and the problem with letting them replicate is that they have a likelihood of mutating and becoming something malicious. So by the power of deduction the correct answer is 3

I disagree with the person who answered before me. For the gene therapy to work the virus must effect all the cells in the body because the mutation will be in every single cell of the patient. If the case were that only a specific cell needs to be fixed (perhapse the mucus destroying enzyme in the lungs) needs to be fixed then yes the previous answer is correct. However if it isn't just one specific thing then the virus will need to be inserted into cells that continue to divide.

The answer is reflection.